Mannan is a non-starch polysaccharide which is a polymer of the monosaccharide mannose. Mannan is found in plant, fungal and bacterial cell walls. Mannan is present in significant amounts in certain plant residues, such as, e.g., softwood.
Softwood is a promising feedstock for bioethanol production. Softwood contains up to 30% mannan in the form of galactoglucomannan. Galactoglucomannans consist of a beta-1,4 linked backbone of beta-D-glucopyranose and beta-D-mannopyranose units, substituted at the C-6 by alpha-D-galactopyranose units. Hemicellulose in softwood may prevent the hydrolysis of cellulose in the absence of accessory enzymes such as hemicellulases. For the complete hydrolysis of mannan-type hemicellulose, a wide array of enzymes is required. The main enzyme involved in hydrolysis of galactoglucomannan is endo-1,4-beta-mannanase (EC.3.2.1.78). Endomannanase cleaves the main chain to oligosaccharides facilitating the solubilisation of galactoglucomannans. The action of endomannanases is restricted by galactose substitutions, hence their cleavage by alpha-galactosidase is needed for the complete hydrolysis of the polymer. Oligosaccharides from galactoglucomannan are hydrolyzed to monomers by beta-mannosidase and beta-glucosidase.
Clarke et al. (Appl. Microbiol. Biotechnol. 53:661-667 (2000)) compare bleaching of softwood paper pulp using combinations of xylanase, mannanase, and alpha-galactosidase.
Varnai et al. (Bioresource Technology 102: 9096-9104 (2011)) disclose that xylanase and mannanase improve the hydrolysis of softwood.
WO 2009/074685 discloses a process of hydrolyzing substrates comprising contacting a slurry of the mannan-containing cellulosic material with an enzyme composition comprising cellulase, mannanase, and mannosidase.
It is an object of the present invention to provide improved processes for hydrolyzing mannan-containing cellulosic materials, e.g., galactoglucomannan and mannan rich softwood substrates.